Induction motors have a copper busbar cage (called a squirrel cage) embedded in the rotor laminations. The torque is created by the stator field interacting with the field created by the currents in the squirrel cage. No currents are generated in the squirrel cage when the rotor RPM is synchronous with the excitation field in the stator. So a 4-pole induction motor has no torque at 1500 RPM (50 Hz), but a lot at 1450 RPM.
Bob S